With the current increase in environmental awareness and the consideration of energy saving economy, the public transportation system has become the major infrastructure with the first priority. In the communication service architecture of the public transportation system, a two-staged service architecture combining the internal-vehicle network with the external-vehicle network is currently and generally applied for providing a whole mobile communication service in a single forward railway carriage to the many passengers.
The radio over fiber (RoF) technique has been widely used in the positions with lower signal receiving qualities, such as a tunnel or a building. Recently, it has further combined the RoF technique with the rail communication system to firstly transform radio signals into optical domain signals to transmit to the remote destination, and to transform the optical domain signals back into electrical domain signals. Thus high frequency signals will not fast fade away in long distance transmission. In the mobile communication, Hand Over/Off is occurred due to the extended range of the base station. The over frequent Hand Over/Off may sharply lower the data transmission efficiency, and even the service is shut down. The influence of the above phenomenon is more obvious in the high-speed mobile communication.
In the rail communication system, the idea of combing the fixed base station with the RoF technique is further proposed due to the characteristic of predictable movements of the public transportation system, and thus a distributed antenna system (DAS) is formed and capable of extending the extended range of the base station. In the meantime, a RF control method is utilized to implement moving cells and avoid the Hand Over/Off. However, the transmitting signals of the RoF technique in the communication system have the same frequency, and a multipath effect may be caused. For the mobile communication system, the multipath effect in the time domain causes the channel variation in the frequency domain, even affects greatly as the multiple paths have similar energy distributions.
For example, the multiple paths having similar energy distributions are easily occurred when the trains moves between two remote antenna units (RAU), thus lowering the receiving qualities, even forcibly shutting down the communication. Combing the moving cells with the service antenna selection is proposed to implement a single line of sight (LoS) signal to lower the multipath effect. For the time domain multiplexing system, directly controlling or selectively turning on the service antenna unit may cause a sudden change of the propagation delay time, hence the uplink signal suffers the timing drift problem because there is no preamble signal to help the synchronization. When errors exceed the receiving capability of the base station, the uplink signal cannot be decoded correctly.
When the distributed antenna system is built, a method for compensating the channels between the base station and the remote antenna units to the same propagation delay time by wiring or adding electrical delay devices is proposed. However, it requires additional fibers or electrical delay devices, and the length of the fibers limit the overall extended range. The propagation delay time of the distributed antenna system includes the signal transmission between the fiber and the air, and the signal is transmitted slower in the fiber than in the air. The longer the length of the used fiber, the smaller the permitted extended range of transmission in the pure air. As it tends to extend the transmission range of the signal, high cost and high density construction is needed; on the other hand, as it tends to improve the extended range of one single antenna, longer fibers cannot be used so that the overall extended range cannot be improved, and thus facing a difficult choice.